1. Field of the Invention
The present invention relates to a process for the preparation of alkylene carbonates by reaction of alkylene oxides with carbon dioxide in the presence of catalysts. Mixed catalysts of the formula EQU a[MX]/b[ZnY.sub.2 ] (III)
which are mixed catalysts of alkali metal halides and zinc halides, are employed as the catalysts. These catalysts can be activated in addition by halogen compounds.
Alkylene carbonates, for example ethylene carbonate or propylene carbonate, are valuable intermediate products for the preparation of plastics, such as, for example, polycarbonates, or they are used as oxyalkylating agents for the synthesis of dyestuffs, plant protection agents or pharmaceuticals. Furthermore, they can be used as solvents, for example in fibre production.
2. Description of the Related Art
It is already known to react alkylene oxides with carbon dioxide in the presence of catalysts to give the cyclic alkylene carbonates. However, in order to achieve industrially adequate rates of reaction, high temperatures and high pressures are required for this process. This presents problems inasmuch as on the one hand the low molecular weight alkylene carbonates in particular arouse industrial interest, but on the other hand the alkylene oxides required for their preparation tend to decompose and therefore require particular safety equipment at high temperatures and under high pressures. High temperatures and high pressures furthermore cause a high expenditure on apparatus, in order to be able to maintain the desired reaction conditions. A large number of compounds have been disclosed to date as catalysts.
Such catalysts include ammonium, phosphonium and sulphonium salts (U.S. Pat. Nos. 2,773,070; 2,994,705; German Offenlegungsschrift 3,244,456), a combination of protic compounds and nitrogen-containing bases (German Offenlegungsschrift 2,611,087), arsonium halides (EP 180,387), tertiary phosphines (WO 84/03701), nitrogen bases (U.S. Pat. Nos. 3,535,341; 3,535,342) and alkali metal halides (BE 872,960). The reaction of alkylene oxides with carbon dioxide in the presence of a catalyst system of a metal chloride and tetraalkylammonium iodides is known from the general chemical literature (Chem. Ber. 119 (1986), 1090, and Chem. Ber. 123 (1990), 277). The reaction of alkylene oxides with carbon dioxide in the presence of catalyst systems comprising an organotin halide and a quaternary phosphonium halide furthermore is known (Bull. Chem. Soc. Japan 60 (1987), 1552). The procedure proposed in these publications comprises saturating a solution of the catalyst in the alkylene oxide with carbon dioxide and carrying out the reaction to give the alkylene carbonate in this manner under normal pressure and at a slightly elevated temperature. Because of the risk of decomposition described for the alkylene oxides, this procedure is unacceptable for safety reasons. The long reaction time (5 hours or more) and the relatively high amount of catalyst used, moreover, are unfavourable. It is furthermore unfavourable that these catalysts in general, as our own experiments have shown (Comparison Examples 3 and 7), soon lose their activity during recycling, which is to be aimed for industrially, and therefore disposal of a large amount of the inactive spent catalyst becomes necessary.